The present disclosure relates to a communications device having a function of adjusting a resonant frequency of a resonant antenna through application of a control voltage to a variable capacitance element, a resonant circuit, and a method of applying the control voltage.
Recently, communications devices, for example, such as non-contact IC (Integrated Circuit) cards used as traffic tickets and electronic money, and information processing terminals having equivalent functions to the non-contact IC cards, have become significantly widespread. In such a communications device, a transmission signal (electromagnetic waves) emitted from a transmitting antenna of a dedicated reader/writer (hereinafter written as “R/W”) device is received by the action of electromagnetic induction in a receiving antenna (resonant circuit) installed inside the communications device.
In the communications device having the above-described non-contact communications function, traditionally, techniques, in which a variable capacitor (variable capacitance element) is provided in a receiving antenna and a resonant frequency of the receiving antenna is adjusted, have been proposed. Further, as such a variable capacitor, for example, a variable-capacitance diode that is called a varicap or a variable capacitor using a ferroelectric material is used.
Further, traditionally, in the resonant antenna having the above-described variable capacitor, various techniques for improving the receiving characteristics have been proposed (for example, see Japanese Patent Application No. 2008-199536 and Japanese Unexamined Patent Application Publication No. 2001-77437).
Japanese Patent Application No. 2008-199536 proposes a technique of improving communications null characteristics through feedback of a DC voltage that is generated based on a signal received from a resonant antenna to a variable capacitor that constitutes a resonant circuit in a non-contact communications device.
Japanese Unexamined Patent Application Publication No. 2001-77437 proposes a technique for eliminating the influence of hysteresis characteristics (control voltage application history characteristics) of a variable capacitor using a ferroelectric material.
By applying a control voltage to a variable capacitor using a ferroelectric material, the capacitance of the variable capacitor is changed. However, the capacitance of the variable capacitor using the ferroelectric material is not determined only by the control voltage that is currently applied, but is affected by the past control voltage application history (hysteresis characteristics). More specifically, the capacitance change characteristics versus control voltage of the variable capacitor when the control voltage is increased from 0 V to a predetermined voltage Vcc differ from those when the control voltage is decreased from the predetermined voltage Vcc to 0 V.
According to Japanese Unexamined Patent Application Publication No. 2001-77437, in order to eliminate the influence of such hysteresis characteristics, the control voltage that is equal to or higher than a saturation voltage of the variable capacitor is once applied to the variable capacitor, and then the variable capacitor is controlled.